Virtual Columns
Virtual columns - such as RA and DEC - do not physically exist in the event file; they are defined by the WCS information attached to another column, e.g. SKY.
For example, looking at any event file:
unix% dmlist acisf01838N001_evt2.fits"[cols sky]" cols -------------------------------------------------------------------------------- Columns for Table Block EVENTS -------------------------------------------------------------------------------- ColNo Name Unit Type Range 1 sky(x,y) pixel Real4 0.50: 8192.50 sky coordinates -------------------------------------------------------------------------------- World Coord Transforms for Columns in Table Block EVENTS -------------------------------------------------------------------------------- ColNo Name 1: EQPOS(RA ) = (+278.3860) +TAN[(-0.000136667)* (sky(x)-(+4096.50))] (DEC) (-10.5899 ) (+0.000136667) ( (y) (+4096.50))
There clearly are columns named "x" and "y." Due to the presence of the "World Coord Transforms" information, you can access the virtual columns RA and DEC as well:
unix% dmlist acisf01838N001_evt2.fits"[cols ra,dec][#row=1:5]" data
--------------------------------------------------------------------------------
Data for Table Block EVENTS
--------------------------------------------------------------------------------
ROW EQPOS(RA,DEC)
1 ( 278.0762532107, -10.6889779897)
2 ( 278.0264043868, -10.6477420690)
3 ( 278.0274740929, -10.6286233286)
4 ( 278.4315818226, -10.4901544415)
5 ( 278.4249218528, -10.5537226939)
